What is this boot option?

Solution 1:

This is the network card in the machine.

PC hardware can boot from block devices (disks, optical media, usb drives) like you are used to, but can also boot using PXE across a network.

PXE uses DHCP to find a server it can boot from, and often uses TFTP to download the boot image.

This is a common way to boot Wyse and Citrix terminals that may have no internal storage.

Solution 2:

It's an Embedded (built-in) Gb (Gigabit capable) NIC (Network Interface Controller).

It's the network port on your motherboard, and it shows up as available in your boot order because of Network Booting.